Order references alphabetically

Signed-off-by: Jonathan Stone <[email protected]>

documents/Specification/MaterialX.PBRSpec.md

@@ -470,16 +470,23 @@ The MaterialX PBS Library includes a number of nodegraphs that can be used to ap
 
 [^Burley2015]: Brent Burley, **Extending the Disney BRDF to a BSDF with Integrated Subsurface Scattering**, <https://blog.selfshadow.com/publications/s2015-shading-course/burley/s2015_pbs_disney_bsdf_notes.pdf>, 2015
 
+[^Chiang2016]: Matt Jen-Yuan Chiang et al., **A Practical and Controllable Hair and Fur Model for Production
+Path Tracing**, <https://media.disneyanimation.com/uploads/production/publication_asset/152/asset/eurographics2016Fur_Smaller.pdf>, 2016
+
 [^Christensen2015]: Per H. Christensen, Brent Burley, **Approximate Reflectance Profiles for Efficient Subsurface Scattering**, <http://graphics.pixar.com/library/ApproxBSSRDF/> 2015
 
 [^Conty2017]: Alejandro Conty, Christopher Kulla, **Production Friendly Microfacet Sheen BRDF**, <https://fpsunflower.github.io/ckulla/data/s2017_pbs_imageworks_sheen.pdf>, 2017
 
+[^d'Eon2011]: Eugene d'Eon et al., **An Energy-Conserving Hair Reflectance Model**, <https://eugenedeon.com/pdfs/egsrhair.pdf>, 2011
+
 [^Georgiev2019]: Iliyan Georgiev et al., **Autodesk Standard Surface**, <https://autodesk.github.io/standard-surface/>, 2019.
 
 [^Gulbrandsen2014]: Ole Gulbrandsen, **Artist Friendly Metallic Fresnel**, <http://jcgt.org/published/0003/04/03/paper.pdf>, 2014
 
 [^Hoffman2023]: Naty Hoffman, **Generalization of Adobe's Fresnel Model**, <https://renderwonk.com/publications/wp-generalization-adobe/gen-adobe.pdf> 2023
 
+[^Marschner2003]: Stephen R. Marschner et al., **Light Scattering from Human Hair Fibers**, <http://www.graphics.stanford.edu/papers/hair/hair-sg03final.pdf>, 2003
+
 [^Oren1994]: Michael Oren, Shree K. Nayar, **Generalization of Lambert’s Reflectance Model**, <https://www1.cs.columbia.edu/CAVE/publications/pdfs/Oren_SIGGRAPH94.pdf>, 1994
 
 [^Pharr2023]: Matt Pharr et al., **Physically Based Rendering: From Theory To Implementation**, <https://www.pbr-book.org/>, 2023
@@ -489,10 +496,3 @@ The MaterialX PBS Library includes a number of nodegraphs that can be used to ap
 [^Walter2007]: Bruce Walter et al., **Microfacet Models for Refraction through Rough Surfaces**, <https://www.graphics.cornell.edu/~bjw/microfacetbsdf.pdf>, 2007
 
 [^Zeltner2022]: Tizian Zeltner et al., **Practical Multiple-Scattering Sheen Using Linearly Transformed Cosines**, <https://tizianzeltner.com/projects/Zeltner2022Practical/>, 2022
-
-[^d'Eon2011]: Eugene d'Eon et al., **An Energy-Conserving Hair Reflectance Model**, <https://eugenedeon.com/pdfs/egsrhair.pdf>, 2011
-
-[^Chiang2016]: Matt Jen-Yuan Chiang et al., **A Practical and Controllable Hair and Fur Model for Production
-Path Tracing**, <https://media.disneyanimation.com/uploads/production/publication_asset/152/asset/eurographics2016Fur_Smaller.pdf>, 2016
-
-[^Marschner2003]: Stephen R. Marschner et al., **Light Scattering from Human Hair Fibers**, <http://www.graphics.stanford.edu/papers/hair/hair-sg03final.pdf>, 2003